There are two main types or categories of electrostatic MEMS actuators. A first type is denoted as gap-closing or parallel-plate MEMS actuators whereas a second type is denoted as an electrostatic comb actuator. As suggested by the name, a parallel-plate actuator includes two or more opposing plates. The plates are configured in the actuator such that a gap between them is closed as one plate is charged positively (or negatively) with respect to the opposing plate. Gap-closing actuators offer considerable actuation force as the electrostatic attraction between two opposite charges is inversely proportional to the square of the separation distance according to Coulomb's law. Thus as the gap separation is reduced towards zero, the electrostatic attractive force is markedly increased. Conversely, the electrostatic attractive force is markedly lowered as the gap separation is increased from zero. Thus, there is a relatively small range of travel for a conventional gap-closing actuator as the plates cannot be pulled too far apart from each other prior to actuation.
In contrast to gap-closing actuators, the separation between fingers in an electrostatic comb does not change. Rather than have the gap change, an electrostatic comb actuator varies the amount of overlap or interdigitation between the comb fingers. This interdigitation can occur over a relatively large range, depending upon the length of the comb fingers. Thus, comb actuators typically offer much better travel than gap-closing actuators. However, since the gap does not close, electrostatic combs are relatively weak in comparison to gap-closing actuators.
There is thus a need in the art for gap-closing actuators that provide the travel advantages of a comb actuator. In addition, there is a need in the art for the control of such improved gap-closing actuators.